WO2006049606A1

WO2006049606A1 - Active automatic tuning for a recharging circuit

Info

Publication number: WO2006049606A1
Application number: PCT/US2004/035981
Authority: WO
Inventors: Marlin H. Mickle; Christopher C. Capelli; Harold Swift
Original assignee: University Of Pittsburgh Of The Commonwealth System Of Higher Education
Priority date: 2004-10-28
Filing date: 2004-10-28
Publication date: 2006-05-11

Abstract

An energy harvesting circuit (6) has an active automatic tuning circuit to search for broadcast frequency in a band of interest and selecting only those broadcast signals received with sufficient RF strength to be used in energy harvesting. This circuit would provide power storage devices with a circuit that has a means to select the ambient RF that can maximize or enhance the performance of an RFID circuit by increasing the amount of energy for harvesting. This automatic tuning would enable a power storage devices charger circuit to move from location to location without manual tuning of the circuit and increase the effective range of an RFID circuit.

Description

ACTIVE AUTOMATIC TUNING FOR A RECHARGING CIRCUIT

BACKGROUND OF THE INVENTION

1. Field of the Invention:

Many radio receivers provide users with a convenience feature for automatically searching for broadcast frequencies in a band of interest and selecting only those broadcast signals received with sufficient quality to be considered "Iistenable" for sustained audio output. This feature, known as search tuning in a "seek", or "scan" (with a truncated period of sustained audio output) operating mode is typically engaged by pressing a button, which then causes the tuner to automatically advance to the next frequency on which a station may broadcast, and evaluate whether a "Iistenable" broadcast is being received. If so, the tuner remains on that frequency. Otherwise, the tuner advances to the next available frequency. 2. Description of the Prior Art:

For example, U.S. Patent 5613230 discloses an AM radio receiver which includes search tuning such as seek or scan tuning features whose stop thresholds for locking onto a received frequency are adapted to environmental conditions. In the preferred embodiment, wideband signal strength, preferably derived from the automatic gain control of the radio frequency amplifier, provides a representative signal to the microprocessor for adapting the thresholds used during search tuning. The thresholds are increased when significant energy is present in the AM band and proportioned to the level of that energy. The decision logic of the microprocessor adapts thresholds according to the tuned signal strength, to wideband signal strength, to nighttime operation during which ionosphere conditions increase transmissibility of distant radio signals, and to intermediate frequency (IF) count validity to reduce the probability of stopping on audibly noisy frequencies when the RF signal environment is noisy. Energy Harvesting

The harvesting of Radio Frequency (RF) energy is extremely important for Radio Frequency Identification (RFID), security monitoring and remote sensing as well as other uses. Some harvesting devices may be tuned to a specific frequency (RFID), while others take advantage of the ambient energy (remote sensing) and others simply sense frequencies for security purposes.

Recharging devices using an RF electromagnetic field radiated into free space have been described. U.S. Patent 6127799 describes a charge storage device that is charged by exposing the charge storage device to an RF electromagnetic field radiated into free space. The charge storage device includes one or more antennas disposed on the device and adapted to receive the radiated RF electromagnetic field. One or more rectifiers are connected to the antennas for rectifying the received RF electromagnetic field into a DC output current. The DC output current produced by the rectifier is used to energize the charge storage device. As disclosed in U.S. Patent 6127799, the antennas may be one or more dipole antennas which are combined to form at least two subsets of dipole antenna element arrays, wherein one subset may be oriented at an acute or a right angle with respect to at least one other subset. The antennas or dipole antennas may be placed on more than one outside surface of the charge storage device which enclose an acute or a right angle with respect to each other. The antennas in the energy harvesting circuit of U.S. Patent 6127799 are designed for a specific frequency.

U.S. Patent Application 10/624,051 by Mickle et al entitled, "Energy Harvesting Circuits and Associated Methods" discloses an energy harvesting circuit that has an inherently tuned antenna, with at least portions of the energy harvesting circuit structured to provide regenerative feedback into the antenna to thereby establish an effective antenna area substantially greater than the physical area

A major problem with the prior art is that the energy harvesting circuits cannot efficiently harvest RF energy at frequencies outside the design specific frequency of the antenna. There is a need for energy harvesting circuits that can harvest RF energy from a broad RF_. spectrum. Furthermore, there is a need for energy harvesting circuits that can harvest RF energy from a broad RF spectrum by having the ability to be tuned to broadcast frequencies (or other ambient frequencies) in a band of interest and select the broadcast signal that maximizes the RF energy. Also, there is a need for small remote power charger device that has a means for receipt of transmitted energy from the environment and energizing power storage devices on an object of interest wherein the power charger device automatically searches for broadcast frequencies in a band of interest and selects the broadcast signal that maximizes the RF energy. SUMMARY OF THE INVENTION

The present invention has met the above-described needs.

In one embodiment of the invention, an energy harvesting circuit has an active automatic tuning circuit to automatically search for broadcast frequencies in a band-of interest and selecting only those broadcast signals received with sufficient RF strength to be used in energy harvesting. This energy harvesting circuit would provide mobile power storage devices with a means to select the ambient RF that can provide the maximum amount of energy for harvesting. This automatic tuning would enable a device with power storage devices to move from location to location without manual tuning of the circuit. The circuit may be formed as a stand-alone unit and, in another embodiment, may be formed on an integrated circuit.

The circuit may include a single broadband antenna that can be tuned using the active automatic tuning circuit selecting the ambient RF that can provide the maximum amount of energy for harvesting. The circuit may include a multiple antenna wherein each antenna is designed for a selected RF spectrum. The active automatic tuning circuit of the energy harvesting circuit would select and tune said antenna or antennas so as to receive select ambient RF that can provide the maximum amount of energy for harvesting.

The circuit may be formed as a stand-alone unit and, in another embodiment, may be formed on an integrated circuit chip. The antenna(s) may take the form of a conductive coil, patch or other type on a planar substrate or may be a stand-alone component.

The invention will be more fully understood from the following description of the invention with reference to the drawings appended hereto. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration of one embodiment of energy harvesting circuit of the present invention that includes a tuning/control system for energy harvesting.

Figure 2 is a schematic illustration of an embodiment of energy harvesting circuit showing a series connection of the circuit to obtain increased voltage.

Figure 3 is a schematic illustration of timing intervals for two different energy accumulation rates.

Figure 4 is a graph illustrating the parameters of feedback signals for maximizing energy harvesting.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one embodiment of the invention, an energy harvesting circuit has an active automatic tuning circuit to automatically search for broadcast frequencies in a band of interest and selecting only those broadcast signals received with sufficient RF strength to be used in energy harvesting. This energy harvesting circuit would provide a mobile battery charger with a means to select the ambient RF signal or combination of signals that can provide the maximum amount of energy for harvesting. This automatic tuning would enable a device with a battery charger circuit to move from location to location without manual tuning of or intervention in the circuit.

The circuit may be formed as a stand-alone unit and, in another embodiment, may be formed on an integrated circuit. The circuit may include a single broadband antenna that can be tuned using the active automatic tuning circuit selecting the ambient RF that can provide the maximum amount of energy for harvesting. Additionally, the circuit may include multiple antennas wherein each antenna is designed for a selected RF spectrum. The active automatic tuning circuit of the energy harvesting circuit would select and tune said antenna or antennas and/or tune the associated circuitry so as to receive select ambient RF that can provide the maximum amount of energy for harvesting. Furthermore, the circuit may be formed as a stand-alone unit and, in another embodiment, may be formed on an integrated circuit chip. The antenna(s) may take the form of a conductive coil, patch, or on a planar substrate, or may be a stand-alone component.

The harvesting of Radio Frequency (RF) energy is, for example, important for Radio Frequency Identification (RFID), security monitoring and remote sensing. Some harvesting devices may be tuned to a specific frequency (RFID), while others may take advantage of the ambient energy (remote sensing) and others simply sense frequencies for security purposes.

The initial area addressed herein is the class of devices that harvest available ambient RF energy. Such devices typically have a broadband antenna covering a part of the total RF band of interest. The most favorable energy sources will vary from location to location such as local AM, FM, TV, WiFi, and from time to time, e.g., FCC regulations before and after sundown.

Therefore, it is desirable to have a tuning mechanism as a part of the harvesting antenna and/or tuner combination to adapt to the most favorable portion of the RF band of interest. While classical communications receivers select single narrow frequency bands to discriminate from other noise or adjacent bands for the sake of clarity, the situation with energy harvesting is somewhat different. The sole function of tuning is to achieve more energy no matter how the incident RF frequencies may be combined, or overlapped. The class of devices being considered includes devices that use batteries or other power storage means such as cell phone, watches, PDAs, for example. Additionally, the class of devices being considered includes remote and unattended devices which may include an automatic mechanism for tuning and adjusting the harvesting antenna/tuner combination while requiring a minimum amount of energy for the tuner operation. As a result, the usable amount of harvested energy is to be reduced by the amount required to operate the tuner thereby providing a very efficient device. Currently, this would suggest that a 24 hour requirement on the order of pica or nano joules would be sufficient for the tuner operation. The embodiment disclosed herein may also be important in applications such as RFID wherein the spectrum is narrow. In such a narrow spectrum, even with a local RF source, the ambient (exogenous) conditions may alter this target frequency whereby the embodiment disclosed would prove to benefit the operation of the RFID device.

The basis of the automatic tuning system is the following:

(a) A feedback concept is employed using a fixed voltage upper limit as the control mechanism to measure the upper limit of time, T, required to obtain_© J energy(t)dt =K

(b) Once the voltage level, K, is reached, the capacitor is discharged onto a general storage unit.

(c) Multiple generator units may be connected in series to provide a higher resulting voltage.

(d) A boost circuit will be used where desired as a DC to DC voltage amplifier. The boost circuit may be (1) commercially available as a device, (2) licensed IP, e.g., Patent No. 6,462,962, (3) a generic circuit, or (4) a new circuit.

One embodiment of a control system is given in Figure 1. In Figure 1, the system consists of a broadband antenna 2, a tuning (or matching) circuit 4, typically LC, with a variable capacitor 6 that can be realized in a number of different ways, a rectifier 10 which is likely a charge pump, a switch 16 that is activated by a specific voltage level that drains the intermediate storage capacitor, and a control circuit 20. The control circuit 20 consists of an oscillator and counter where the counter has a buffer to store count i to be compared with count i + 1. Thus, the following conditions can occur:

(1) Count i < Count i + 1, where state i is preferred to state i + 1

(2) Count i > Count i + 1 , where state i + 1 is preferred to state i (3) Count i = Count i + 1, which implies no action It may also be desirable to extend the single measurement to a window of time say i-k, i-k+1 ,...i and i+1 , i+2,...i+k whereby an average would be obtained to reduce unnecessary "hunting" of the circuit.

The system of Figure 1 maybe connected as multiple systems 30,32,34 as shown in Figure 2 in order to achieve a higher voltage.

The switch of Figure 1 is a member of a class of solid state switches available in a wide variety of voltages.

The variable capacitor, C, of Figure 1 can be implemented in a variety of ways including (1) using transistors as capacitors, (2) using multiple external discrete capacitors with solid state switches, and (3) using capacitors fabricated on the same chip to be included or excluded in the circuit by using switching transistors.

The ambient energy will vary giving a rate of change of accumulated energy as shown in Figure 3. The time to reach a certain voltage where joules = 1/2 (C*V²) will vary depending on the energy available during a specific period of time, say Tj. Figure 3 is an illustration of two different periods of time, T₃ and T_b, that are required to accumulate the same amount of energy, 1/2 (C* V²).

By counting the intervals between pulses for a fixed value of V, it is therefore possible to differentiate two conditions of accumulated energy. In Figure 3, case a, T_a is better (8 pulses) than case b, T_b (11 pulses) because it takes less time to reach the value of energy, 1 /2 (C* V²).

Figure 4 illustrates the situation where an ambient voltage source is held constant and the capacitor is switched to show the effect on the charging time to reach the voltage 1/2 (C* V²).

Whereas particular embodiments have been described herein for purposes of illustration, it will be appreciated by those skilled in the art that numerous variations of the details may be made without departing from the invention as defined in the appended claims.

Claims

We Claim

1. A remote station for receipt of ambient energy from the environment and energizing energy storage devices of objects of interest comprising: at least one antenna; circuitry for converting said energy into DC energy for energizing said power storage devices; and circuitry for automatic search tuning to tune said antenna.

2. The remote station of Claim 1 wherein the ambient energy is RF energy.

3. The remote station of Claim 1 wherein the circuitry for automatic search tuning to tune said antenna to a frequency that provides generally the maximum RF ambient energy that can be harvested for a specific location.

4. The remote station of Claim 1 including said circuitry having multiple circuit elements connected in series.

5. The remote station of Claim 1 including said circuitry having a control system which includes a broadband antenna, a tuning circuit, a rectifier and a switch.

6. The remote station of Claim 5 including said tuning circuit including an LC circuit with a variable capacitor.

7. The remote station of Claim 6 including said rectifier being a charge pump.

8. The remote station of Claim 6 including said switch being structured to be activated by a specific voltage level.

9. The remote station of Claim 5 including said control system having a control circuit.

10. A remote station for receipt of ambient energy from the environment . and energizing energy storage devices of objects of interest comprising: at least one antenna; circuitry for converting said energy into DC energy for energizing said power storage devices; and circuitry for automatic search tuning to tune said circuitry.

11. A remote station for receipt of local energy for an RFID implementation from the environment and energizing energy storage devices of objects of interest comprising: at least one antenna; circuitry for converting said energy into DC energy for energizing said power storage devices; and circuitry for automatic search tuning to tune said antenna.

12. A remote station for receipt of local energy for an RFID device from the environment and energizing energy storage devices of objects of interest comprising: at least one antenna; circuitry for converting said energy into DC energy for energizing said power storage devices; and circuitry for automatic search tuning to tune said antenna.

13. The remote station of claim 1 for receipt of ambient energy from the environment and energizing energy storage devices of objects of interest comprising: at least one antenna; circuitry for converting said energy into DC energy for energizing said power storage devices; circuitry for automatic search tuning to tune said antenna, and means for measuring the voltage to determine the energy harvesting effectiveness of said ambient energy remote station.

14. The remote station of claim 11 for receipt of ambient energy from the environment and energizing energy storage devices of objects of interest comprising: at least one antenna; circuitry for converting said energy into DC energy for energizing said power storage devices; circuitry for automatic search tuning to tune said antenna, and means for measuring the voltage to determine the energy harvesting effectiveness of said RFID implementation.